Two 1000s create a ton of heat so it'll be a bit of a battle for you unfortunately. I still say take the filter off the one run and see how much that helps since the fan will be able to pull air out faster.
Th other thing you could do wold be to try and figure out a way to get the run with the lights in it to pull that cooler basement air through the lights instead if the air inside the tent that is being heated by them. The lights will run cooler which will help things a bit. I know you said there's no vent on that side but there's got to be a way to get it. I'm not suggesting you cut your tent but some people do cut a 6" hole and run a duct out that hole so the air goes straight through the lights and back into the room the tent is in.
Do the temps in the basement get warm towards the middle or end of the lights on part of the cycle? If so then Don is on to something. My tents exhaust into a 10x12 room but that room gets exhausted to the outside after the air is deodorized. The exhaust for the entire room is what helps manage temps in my case but I still run my lights at night and have the 12 hrs of dark be during the day. I recently installed A/C in the 10x12 too but it's mainly to prepare for summer months and the units have not needed to turn on yet. My temp monitor says it hasn't been hotter than 78f and I set the A/C at 80f. I'd like to run the room and tents at about 75f but I don't want to commit to the extra electricity right now since I'm already burning three 600s, a 1000 and for veg there's a t8 flouro that's almost 200w. There's a few fans running too
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Good luck and keep me posted as to how it works out.